It has been well known that a wholly aromatic polyamide produced from an aromatic diamine and an aromatic dicarboxylic dichloride is excellent in heat resistance and excellent in flame retardancy. It has also been known that the wholly aromatic polyamide is soluble in an amide solvent, and a fiber can be obtained from the polymer solution through such a method as dry spinning, wet spinning, semi-dry and semi-wet spinning and the like.
In the wholly aromatic polyamide, a fiber of a meta-type wholly aromatic polyamide (which is hereinafter abbreviated as “metalamide” in some cases) represented by poly-m-phenylene isophthalamide is particularly useful as a heat resistant and flame retardant fiber, and as a method for producing the metalamide fiber, the following two methods (a) and (b) have been employed. Furthermore, the following method (c) to (e) have been also proposed as a method for producing the metalamide fiber.
(a) A method, in which m-phenylenediamine and isophthalic chloride are subjected to low temperature solution polymerization in N,N-dimethylacetamide to prepare a poly-m-phenylene isophthalamide solution, and then a polymer solution containing calcium chloride obtained by neutralizing hydrochloric acid by-produced in the solution with calcium hydroxide is dry-span to produce a metalamide fiber (see JP-B-35-14399 and U.S. Pat. No. 3,360,595).
(b) A method, in which an organic solvent (such as tetrahydrofuran) that is not a good solvent of a product polyamide containing a m-phenylenediamine salt and isophthalic chloride is made in contact with an aqueous solution system containing an inorganic acid receiving agent and a soluble neutral salt to isolate powder of a poly-m-phenylene isophthalamide (see JP-B-47-10863), and the polymer powder is re-dissolved in an amide solvent and then wet-spun in an aqueous coagulation bath containing an inorganic salt to produce a fiber (see JP-B-48-17551).
(c) A method, in which a molded article, such as a fiber, is produced by a wet-molding method from a metalamide solution containing no inorganic salt or a small amount (2 to 3%) of lithium chloride formed by dissolving a metalamide synthesized by a solution polymerization method in an amide solvent (see JP-A-50-52167).
(d) A method, in which a metalamide polymer solution, which is formed by solution polymerization in an amide solvent and contains calcium chloride formed by neutralizing with calcium hydroxide, calcium oxide or the like, and water, is passed through a gas by extruding into the gas through a spinning die, and then introduced into an aqueous coagulating bath, and is further passed through an aqueous solution of an inorganic salt, such as calcium chloride and the like, to form into a fiber material (see JP-A-56-31009).
(e) A method, in which a metalamide polymer solution, which is formed by solution polymerization in an amide solvent and contains calcium chloride formed by neutralizing with calcium hydroxide, calcium oxide or the like, and water, is spun into an aqueous coagulation bath containing calcium chloride in a high concentration through a spinning die to form into a fiber material (see JP-A-8-074121 and JP-A-10-88421).
(f) A method, in which a polymer solution of an amide solvent is ejected into a spinning column at a high temperature through a spinning die, and cooled with an aqueous solution at a low temperature at the time of being ejected from the spinning column, and is then stretched in a plastic stretching bath to form into a fiber having a considerably fine porous nature and a density of 1.3 g/cm3 or less (see JP-A-52-43930).
(g) A method, in which a meta-type aramid polymer solution containing substantially no salt is coagulated by ejecting into a coagulation bath containing an amide solvent and water to form a fibrous material (filament), which is subsequently stretched in a plastic stretching bath containing an amide solvent and water and then subjected to rinsing with water and a heat treatment (see JP-A-2001-303365, JP-A-2003-301326, JP-A-2003-342832, JP-A-2004-3049, JP-A-2005-54315 and JP-A-2005-54335).
(h) A method, in which a meta-type aramid polymer solution is coagulated by ejecting into a coagulation bath containing an amide solvent and water to form a fibrous material (filament), which is subsequently stretched in the air under heat in a state where the fibrous material contains the amide solvent, and then subjected to heating and a heat treatment (JP-A-2001-348726).
(i) A method, in which a polymer solution containing a meta-type aramid and an amide solvent containing a salt is ejected into a coagulation bath containing an amide solvent and water and containing substantially no salt to form a porous filament, which is subsequently stretched in a plastic stretching bath containing an aqueous solution of an amide solvent and subjected to rinsing with water and then a heat treatment (JP-A-2005-232598).
In the method (a), in the fibrous polymer solution spun from the spinning die, the solvent is vaporized and dried from the vicinity of the surface of the fibrous material to be formed owing to dry spinning, and thus a dense and firm skin layer is formed on the surface of the fiber. Accordingly, it is difficult to remove sufficiently the solvent remaining in the fibrous material after spinning even by rinsing the solvent with water or the like. Consequently, the solvent remaining in the fiber brings about such a problem that the fiber suffers, upon using in a high temperature atmosphere, yellowing and generation of an organic gas due to evaporation or decomposition of the solvent remaining.
In the methods (b) to (e), evaporation of the solvent does not occur in the spinning step owing to wet spinning, but upon introducing into the aqueous coagulation bath or the aqueous coagulation bath containing an inorganic salt in a high concentration, the solvent is released from the vicinity of the surface of the polymer solution formed into a fiber form into the aqueous coagulation bath, and simultaneously water invades into the interior of the coagulated fibrous material from the vicinity of the surface thereof, so as to form a firm skin layer. Accordingly, the problem of yellowing and organic gas due to the solvent remaining in the fiber cannot be avoided as similar to the fiber formed by the dry spinning method.
The method (f) is for producing a porous meta-type aramid fiber having a density of 1.3 g/cm3 or less, but is an application technique of the dry spinning method, and thus has the similar problem as in the dry spinning method having been described.
In the method (g), it is important to use a salt-free meta-type aramid polymer solution containing substantially no salt, and there is such a problem that for producing the salt-free polymer solution, such a step is required in that the polymer is isolated to rinse and remove a salt, or a salt in the polymer solution is filtered with a filter or the like.
In the method (h), the stretching is carried out in the air, and thus it is difficult that the temperatures of the filament and the amounts of the solvent and the like remaining each are identical between the outer periphery and the core part of the filament, whereby it is difficult to obtain a homogeneous meta-type aramid fiber.
In the method (i), the molecular orientation of the fiber is increased by stretching in the plastic stretching bath after coagulation, but the orientation is liable to be relaxed in the water rinsing and/or warm water rinsing step. Accordingly, it is necessary to increase the orientation again in the heat treatment step for obtaining a fiber having high strength, and on the other hand, rapid crystallization occurs in the heat treatment step. Consequently, the resulting fiber has such a problem that it has a high heat contraction rate at 300° C. or more.
JP-A-2000-303365 proposes a method for producing a dense metalamide fiber containing substantially no salt (inorganic ionic substance), in which a polymer solution containing no salt formed by dissolving a metalamide having m-phenylenediamine isophthalamide obtained in the similar method as the method (b) as a major repeating unit, in an amide solvent, is ejected into a coagulation bath containing an amide solvent and water and containing no salt, to coagulate as a porous fibrous material (filament), which is subsequently stretched in a plastic stretching bath containing an aqueous solution of an amide solvent, followed by subjecting to rinsing with water and then heat treatment. JP-A-2001-348726 proposes a method, in which a porous fibrous material thus coagulated is stretched under heat in the air in a state where the pores contain the coagulation liquid or a plasticizing liquid, and heated in a state where the pores contain the coagulation liquid, followed by subjecting to heat treatment.
In the methods, however, a porous fibrous material having substantially no skin layer on the surface can be obtained in the step where the metalamide solution is formed in to the fibrous material by coagulation. However, upon removing sufficiently the solvent remaining in the fiber in the state of the porous fibrous material, it is significantly difficult to stretch thereafter, and thus it is difficult to effect sufficient orientation and crystallization. Accordingly, the fiber thus obtained by the methods has such a defect that it is inferior in fiber property that is inherent to a metalamide fiber, particularly high temperature heat contraction stability.
As having been described, it is the actual current situation that such a meta-type wholly aromatic polyamide fiber has not yet been known that is suppressed in yellowing and generation of an organic gas in a high temperature processing step, and is capable of providing a product having high performance, such as flame retardancy, breaking strength and dimensional stability in a high temperature atmosphere.